PSI - Issue 18

Natalia Kosheleva et al. / Procedia Structural Integrity 18 (2019) 129–134 Author name / Structural Integrity Procedia 00 (2019) 000–000

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sense the changes of the environmental conditions and to assess their mechanical state and even react to these changes. Among the most promising are the fiber-optic sensors which in combination with widely spread composite materials open new prospects in design and manufacturing of objects of various types of purposes. Thus, it becomes possible to monitor such parameters as strain, temperature and other parameters related to monitoring the state of the structure. Despite its small size, the diameter of the optical fiber is comparable to the size of a layer of composite material and, if embedded, can cause distortion of the layers. During the manufacturing process of a composite material, this area around the optical fiber becomes filled with epoxy resin. Such a technological defect in shape resembles the eye and in the literature is called the resin pocket or resin rich area (Di Sante 2015). Such technological defect can be dangerous due to the high stress concentration and, hence, the initiation of primary damage in the structure. The size and shape of the resin pocket are determined by the mechanical properties of the composite material, the number and layout of the layers and the size of the optical fiber. In the study of (Lammens et al. 2015) an approach to determining the shape of a resin pocket, based on numerical finite element modeling of the pressing process in a plane-strain formulation of the theory of elasticity problem, is considered. Verification of the obtained numerical results with real cross-sections images of polymer composite material (PCM) samples with embedded optical fiber showed the reliability of this method for determining the shape of the resin pocket. The work (Al-Shawk, Tanabi, and Sabuncuoglu 2018) investigated the effect of the resin pocket geometry, caused by the introduction of the microvascular canal into glass fiber reinforced plastic (GFRP), on the concentration and stress distribution during the failure of composites with two types of stackings [0/90] 4s and [90/0] 4s . A goal of the study of (Silva et al. 2005) was a quantitative and comparative assessment of the effect of embedded optical fibers on the mechanical behavior of carbon fiber reinforced plastic (CFRP). It was tested whether the presence of optical fibers could cause the degradation of the mechanical characteristics of the receiving material in three types of mechanical tests: impact tests, static bending tests and fatigue tests. Most of the well-known studies focus on the research of the resin pocket in composite materials with a unidirectional reinforcement structure of each layer. In this case, it is known that the size of the resin pocket depends on the orientation of the optical fiber with respect to the reinforcing fibers of the layers between which the optical fiber is embedded. The most significant distortion of the structure of the host material takes place when the optical fiber is embedded at an angle of 90 degrees to the direction of reinforcement of the layers of the composite material (Shivakumar and Emmanwori 2004). However, for composite materials with a woven layer reinforcement scheme, the internal structure of the composite is significantly different. And the embedding of such a foreign object as an optical fiber can cause a significantly different, as compared to unidirectional composites, change in the internal structure. The main objective of this work was to conduct an experimental study of the internal structure of CFRP and GFRP composite samples with woven reinforcement and with embedded optical fiber based on the study of cross sections using an optical microscope. The aim of the work was to identify the appearance of defects caused by the integration of fiber-optic strain sensors (FOSS) into the material, and, if necessary, to assess their geometrical characteristics. 2. Technological process of sample manufacturing with embedded optical fiber The fiber reinforced polymer composite material consists of reinforcing fibers and the polymer matrix. A woven fabric with weave style 2  2 twill was chosen as the reinforcement material, with the weaving pattern shown in the Fig. 1. Further, this material was impregnated with resin to obtain a prepreg. The process of creating products from polymer composite materials includes several basic steps: laying out a reinforcing material in the specialized form, package assembly, the polymerization process and removal of manufactured sample. These technological steps are present in each manufacturing technique for PCM with some additional items depending on the particular type. For this study, one of the most common PCM manufacturing techniques - autoclave moulding was used.